Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardiovertors, and defibrillators, for example, require a reliable electrical connection between the device and a region of the heart. Typically, a medical electrical "lead" is used for the desired electrical connection.
One type of commonly used implantable lead is a transvenous lead. Transvenous leads are positioned through the venous system to attach or electrically connect at their distal end to the heart. At their proximal end, they are connected to typically an implantable pulse generator. Such leads normally took the form of a long, generally straight, flexible, insulated conductor. Among the many advantages of a transvenous lead is that it permits an electrical contact with the heart without physically exposing the heart itself, i.e., major thoracic surgery is not required.
The specific design of a transvenous lead used is often varied depending upon the region of the heart to which it is to be connected. For example, U.S. Pat. No. 4,402,330 of Lindemans discloses a body implantable lead in which the lead body has a J-curve and the distal electrode has a permanent bend. In such a manner, the lead is configured to electrically connect to the right atrium.
While such a lead has been found acceptable for electrically connecting and thus pacing the right atrium, the need exists for a transvenous medical electrical lead which may provide an electrical connection to the left atrium. Of course the left atrium cannot, at present, be transvenously accessed with a lead for chronic implantation due to the direction of blood flow and the present limitations of materials. To be precise, blood flows through the right side of the heart (atrium and ventricle), through the lungs, through the left side of the heart (atrium and ventricle) and then through the rest of the body, including the brain, before returning again to the right side of the heart. Implanted objects, however, often cause minor blood clots and thrombus to form in the blood. These may, on occasion, dislodge and be released into the bloodstream. Because the blood circulates directly from the left atrium and ventricle to the brain, any clots, however minor, could have serious consequences if they were to reach the brain, e.g. a stroke. In contrast, any clots released from an object implanted in the right side of the heart would simply travel to the lungs, where they would lodge without any serious risk. Thus at present, chronic transvenous leads may not be safely implanted within the left side of the heart.
In spite of the difficulties, there remains a great need to be able to electrically stimulate or sense or both the left side of the heart. The most obvious reason is the left side of the heart accounts for the majority of the heart's hemodynamic output. For example, the left ventricle has a greater wall thickness (10-20 mm as compared to 1-5 mm) than the right side. This, of course, is reasonable given that the left side of the heart must pump blood throughout the body while the right side only pumps blood through the lungs.
Because the left side is relatively more important for hemodynamic output, not surprisingly various pathologies may be better treated through stimulation on the left side of the heart. For example, in patients with dilated cardiomyopathy, electrical stimulation of both the right side and the left side of the heart has been shown to be of major importance to improve the patients well-being and manage heart failure. See, for example, Cazeau et al., "Four Chamber Pacing in Dilated Cardiomyopathy," PACE, November 1994, pgs. 1974-79. See also Brecker and Fontainem, St. et al., "Effects Of Dual Chamber Pacing With Short Atrioventricular Delay In Dilated Cardiomyopathy," Lancet November 1992 Vol. 340 p1308-1312; Xiao HB et al., "Effect Of Left Bundle Branch Block On Diastolic Function In Dilated Cardiomyopathy," Br. Heart J 1991, 66(6) p443-447; and Fontaine G et al, "Electrophysiology Of Pseudofunction," CI.Meere (ed.) Cardiac pacing, state of the art 1979, Pacesymp, 1979 Montreal.
At present there are several techniques for implanting a lead onto or into the left side of the heart. First, of course, is through general thoracic surgery; either via a median sternotomy; intercostal approach; or, in a more limited procedure, a subxiphoid approach. These procedures, however, involve major surgery which may be painful and dangerous for the patient, as well as extremely costly. The sub-xiphoid approach, moreover, only permits limited access to the anterolateral surface of the left ventricle and does not provide any access to the left atrium. Another approach used is to electrically access the left atrium is through the coronary sinus.
The coronary sinus, however, presents challenges in both implanting the lead in the proper position as well as ensuring the lead maintains sufficient electrical contact with the desired tissue. U.S. Pat. No. 5,423,772 of Lurie et at. discloses a coronary sinus catheter having three sections. Each section has varying degrees of flexibility, with the proximal reinforced section being stiffer than an intermediate section, the intermediate section being stiffer than the softened tip section. The catheter also is curved, with the curve beginning in the intermediate section, the curve further continuing into the softened tip section, where the radius of curvature decreases, i.e., the catheter becomes more curved closer to the tip. One drawback to such a design, however, is that the particular shape of the curve is not ideally suited for electrically accessing the left atrium. In addition, such a catheter is relatively complicated to manufacture due to the required reinforcing braid or other mends in the proximal reinforced section. Finally, such a catheter does not permit introduction of a stylet to assist in the placement of the catheter into the coronary sinus.
It is thus an object of the present invention to provide a medical electrical lead which is suitably shaped to provide an electrical connection through the coronary sinus to the left atrium or even the left ventricle.
A still further object of the present invention is to provide such a medical electrical lead which may be readily flexed during implantation to provide the ability to be introduced transvenously.
A still further object of the present invention is to provide a medical electrical lead having an electrode which may be securely contacted against the coronary sinus wall but which will not occlude the coronary sinus.